German Patent Document No. DE 195 45 064 A1 describes a cassette filter which is used as a particulate filter and as a microfilter. Cassette filters of this type have proven successful in practice. Pleated filter mats such as those disclosed in German Patent Document No. DE 195 45 046 C2 are used as the filter medium.
The filter media used with cassette filters are of various designs and material compositions. For example, glass fiber paper is used for filters of higher filter classes, but it has a very low resistance to mechanical stresses. It tears easily, in particular when handled improperly, which results in damage to the filter medium, and thus to an impairment and possibly even to loss of the filter properties. For this reason, in a number of applications, the filters are provided with a protective grid in front of and/or behind the filter package to protect the filter medium from contact and/or damage. The protective grid is glued to and/or embedded in the filter package in a frame. The protective grid, which is also referred to as a grip guard, must have a high air permeability combined with a high rigidity at the same time. The material to be used must be selected accordingly. Since the filters are sometimes very limited in their overall height, the material must not be too thick. In addition, the grip guard should have a planar design. To meet this condition, protective grids are currently being made of expanded metal, stainless steel, or aluminum. However, this material substantially increases the weight of the filter. In addition, the material is not incinerable, i.e., it cannot be reduced to ashes. One possibility for making it incinerable might be to use protective grids made of plastic instead of metal. However, experiments have shown that the rigidity of such grids is not sufficient to reliably prevent damage to the filter medium.